The distance over which data can be transmitted in optical fibers is limited by optical power loss and spectral pulse dispersion. With the advent of erbium-doped optical fiber amplifiers this limitation has been virtually eliminated, particularly for optical communications systems operating in the 1550 nm band. To compensate for power loss and dispersion, a compensating optical fiber, as part of an amplification and/or transmission system, is typically wound on a spool and the spool is used as-is or is placed in a housing. Leads are attached to the end of the optical fiber for connecting to the optical communications systems. This entire device may be described as a “dispersion compensation module.”
Presently, fiber-based dispersion compensating modules made at Corning Incorporated and other manufacturers utilize a length of dispersion compensating fiber, for example, an erbium-doped fiber that is wound on a spool. Some spools are molded and cost effective, but most spools are made by attaching steel or aluminum flanges to each side of a hub. This process of spool assembly involves the costly assembly of custom parts, the exact configuration of which depends on the customer requirements. Consequently, different spools must be designed and stored for each customer. The spool is then installed in a box enclosure or, alternatively, a protective band is placed around the outer diameter of the spool to protect the fiber and the spool is used as is. (Either configuration may be termed a dispersion compensation module.) A typical spool assembly of the prior art is illustrated in FIG. 1.
When the typical spool such as that in FIG. 1 is wound under tension with dispersion compensating fiber (DCF), a “buffer layer” of optical fiber is first laid on the hub of the spool. The exact number of layers may vary, but generally fall in the range of 5–10 layers. This buffer layer will not be connected to or utilized by the optical communications system. The purpose of the fiber buffer layer is to form a protective layer for the fiber that is actually being used in the product DCF (the “operating fiber”) from falling into the crevice that can occur between the flange and the hub of the spool. In addition, the buffer layer serves to protect the DCF from the hub during thermal excursions in which the hub will expand at a different rate than the DCF. If the operating fiber were not so protected it would rub on the hub at that location and could be damaged, resulting in a failed dispersion compensation module. Consequently, while the buffer layer is a necessary item in the typical winding operation of a spool assembly, it adds cost and complexity to the finished product.
Once the spool is wound, the optical fiber on the spool remains at some level of tension. This tension is believed to degrade the optical properties of the fiber over time. In addition, during thermal excursions, whether in manufacturing, testing, or field use, the fiber can be further stressed due to thermal expansion effects as the hub expands more than the fiber pack. This can cause further optical problems, and in a worst case, reliability issues such as fiber breakage can occur.
As a result of the foregoing problems, there exists a need for a dispersion compensation module which does not rely on a spool to hold the fiber and does not require the use of a buffer layer of costly fiber to protect the operating DCF. There is also a need for a dispersion compensation module in which the DCF if not under tension or in which the tension has been sufficient relaxed so that stress-induce problem doe not arise in the fiber with the passage of time.